1. Field of the Invention
The invention relates to a speech path switch used in a telephone exchange or the like and, more particularly, to a speech path switch fabricated in a semiconductor integrated circuit.
2. Description of the Prior Art
A speech path switch is generally used in a telephone exchange for coupling a subscriber with another subscriber. A conventional speech path switch uses a mechanical contact for the crosspoint. This mechanical type crosspoint encounters problems in miniaturizing the size of the exchange, reducing the weight and improving the reliability of the exchange. An integrated semiconductor speech path switch using semiconductor switching elements for the crosspoints had been developed with the view of overcoming the just-mentioned problems.
The semiconductor speech path switch for the telephone exchange must permit high voltage analog signals such as a bell signal and metering pulses to pass therethrough. For this, the integrated circuit elements must withstand extremely high voltage and current, thus needing large surface areas of the elements. The necessity of the bidirectional function results in increasing the number of elements used. Generally, the speech path is constructed by a two-wire balanced path, so that the number of terminals is increased.
Therefore, it is very difficult to manufacture a balanced two-wire, 8.times.8 matrix speech path switch usually used as a minimal matrix by the monolithic integrated circuit technology. Thus, such a minimal 8.times.8 matrix requires multi-chip packaging.
However, if the multi-chip packaging on a wiring substrate is employed, it is difficult to fabricate a matrix of 8.times.8 into one package in view of the number of chips, the number of package terminals, the size of a substrate, and the manner of wiring, sealing and the like. Therefore, in the past, a 4.times.4 matrix was constructed by one package and an 8.times.8 matrix must be formed on a large printed circuit board, as disclosed in U.S. Pat. No. 3,735,057.
Further, in constructing a grid during subsequent packaging operations by mounting a plurality of speech path switches on a printed circuit board, it must withstand a high voltage signal of 100 V or more, with low wiring resistance. This necessitates a relatively larger space for the speech path wiring on the printed circuit board, as compared with the switch handling signals of logical level. Particularly, it is impossible to dispose the speech path wiring between usual IC terminals (spaced by 2.54 mm, for example). For example, in the case of a speech path switch with a 200 V voltage withstand requirement, at least 2.9 mm is necessary for the space between the IC terminals.